Common area indoor commercial paging systems (non-fire alarm) have used a “constant voltage” (25, 70 or 100 volt) technology for decades. Briefly, this technology allows easy distributed ceiling or wall speaker design that involve speakers that use “matching transformers” for each and every speaker in the system. These transformers permit easy calculation of how much power is needed for adequate volume in a given area.
For example, if a system consists of twenty speakers, and an adequate power for each speaker is one watt, then the driving power amplifier would have to provide at least twenty watts to adequately handle twenty speakers. Typically, though, it is more desirable to have a larger amplifier, say 50 or 100 watts, to accommodate for speakers that sound in a larger area and may require more than one watt for good sound level coverage.
This is where the transformer comes into play. Commonly, the speaker's transformer has multiple connections, or “taps,” that range from ⅛ watt to as much as 30 or more watts. Again, just adding up the wattage for the system determines the size of the power amplifier that will drive it.
This same speaker technology has generally been adopted for use with audio (voice) fire alarm systems that utilize speakers to alert occupants of a building of an emergency. The warning typically consists of alert tones followed by spoken word messages that give instructions to occupants during the emergency.
The disadvantage to using this technology lies in the setting of each speaker's transformer taps. If it is deemed that a particular speaker is not loud enough in a given area, the service technician must remove the speaker from the wall or ceiling, move the tap connector to the next higher tap setting, re-install the speaker and then test the output, usually with a dB meter, to see if the audio is now loud enough. (NFPA 72 “National Fire Alarm Code” requires that speakers used in fire alarm systems produce a sound that is at least 15 dB above the ambient noise level of a given area).
This is, more or less, a trial-and-error method of setting speaker loudness, and may have to be repeated several times. One of the biggest factors in determining proper dB levels lies in the actual construction material of the area in question, and the anticipated ambient noise level. There are methods to predict the required dB level before installation, but it is cumbersome and expensive to make this prediction, particularly if there are unknowns involved, usually in new construction situations. Thus, these predictive methods are not widely used for fire alarm systems.